This invention relates to powder spraying systems, and, more particularly, to a system for applying particulate powder material in accurately metered quantities onto a target surface.
Powder spray systems for applying particulate powder material onto a substrate generally include a powder pump for transporting air-entrained powder from a fluidized bed powder container through a pneumatic conveyor line to a powder spray gun. A conventional open top fluidized bed powder container includes four sidewalls, a bottom wall and an air-pervious wall spaced above the bottom wall which supports the particulate powder material. A chamber is formed between the air-pervious wall and the bottom wall into which high pressure air is introduced through a fluidizing air line. The pressurized fluidizing air passes into the air chamber and then moves upwardly through the pervious wall to fluidize the powder, i.e., make it readily flowable, for delivery through the pneumatic conveyor line to the spray gun.
In many applications, a venturi-type powder pump is mounted within the container which pumps the fluidized powder into the pneumatic conveyor line for delivery to the powder spray gun. Venturi powder pumps include a siphon tube which extends into the fluidized powder within the container, and a venturi pumping chamber connected to the siphon tube. High pressure air is directed through a delivery air line into the venturi pumping chamber and in the course of passage therethrough creates a negative pressure within the venturi pumping chamber which sucks or draws powder from the container through the siphon tube and into the pumping chamber.
Venturi powder pumps also commonly include a metering or atomizing chamber connected to the siphon tube. High pressure atomizing air enters the atomizing chamber through an atomozing air line and impacts the fluidized powder being drawn through the siphon tube by the delivery air line. The atomizing air controls the amount of air mixed with the powder flowing to the venturi pumping chamber to obtain the desired feed rate of powder material flowing into the pneumatic conveyor line for delivery to the spray guns.
Powder spray systems employing metered venturi powder pumps therefore require three separate air lines to deliver particulate powder material from a container to a spray gun, including a fluidizing air line to fluidize the powder within the container, a delivery air line to draw the fluidized powder out of the container into the pump and an atomizing air line to meter or atomize the powder before it exits the pump. Powder spray systems of this type require a substantial quantity of air to operate all three lines. In addition, the pressure in each air line is generally not the same. This requires some type of air supply system capable of providing different air pressure to each of the fluidizing, delivery and atomizing air lines. One such system is disclosed, for example, in U.S. Pat. No. 4,640,310, filed Dec. 26, 1984 and entitled, "Variable Air-Piloted Air Regulator System", which is owned by the assignee of this invention. Air supply systems of this type, although effective, add cost to the overall powder spray system.
Another problem with powder spray systems employing venturi powder pumps is that the volumeric capacity of venturi powder pumps is relatively modest, i.e., on the order of about 20 grams per second. Some applications require additional capacity which cannot be accommodated by standard venturi powder pumps.
A further problem with venturi powder pumps is the difficulty in obtaining a consistently accurate feed rate of particulate powder material, particularly in applications requiring "stitching" or intermittent application of powder onto a substrate. As mentioned above, the powder/air ratio in the air-entrained powder stream is primarily determined by the air pressure in the atomizing air line, and, to a lesser extent, in the fluidizing and delivery air lines. The air pressure within such lines can be controlled with reasonable accuracy, but some fluctuations do occur particularly when the spray gun is operated intermittently with the result that the feed rate of powder delivered to the spray gun can be inconsistent. This is unacceptable in some applications, such as the application of particulate powder adhesive material onto the non-woven fabric materials used in the fabrication of disposable diapers, where the quantity of adhesive material applied to the non-woven fabric must be controlled with high accuracy. If too little adhesive material is applied to the non-woven fabric, for example, the finished diaper may not have the required tensile strength. Too much powder adhesive material applied to the non-woven fabric might result in a diaper having less softness or fluffiness than desired.
Another problem with powder spray systems employing venturi powder pumps involves the physical size of such systems. It has been found that particulate powder material tends to collect in the delivery lines or hoses between the venturi powder pump and spray gun if the hoses are relatively long. This creates a problem known as "puffing" in which the powder collects in the hose and creates a pressure buildup or back pressure in the hose. When the back pressure is sufficient, a relatively large quantity of the collected powser is suddenly forced into the spray gun at one time creating an undesirable surge of powder onto the substrate. In order to avoid "puffing", the venturi pumps must be located close to the spray gun to shorten the length of the hoses as much as possible. But in many diaper lines, for example, there is no room to position the spray gun, venturi pump and powder container immediately adjacent the line. Redesign of the diaper line to provide additional space is expensive, if it is feasible to do at all.